In recent years, digital mobile communication systems for fully digital speech and data transmission have been introduced. As far as the mobile communication network is concerned, the most limited resource is the radio path between the mobile stations and the base stations. To reduce the bandwidth requirements of a radio connection on the radio path, speech transmission utilizes speech encoding providing a lower transmission rate, for example 16 or 8 kbit/s instead of the 64 kbit/s, transmission rate typically used in telephone networks. For speech encoding, both the mobile station and the fixed network end must have a speech encoder and a speech decoder. On the network side, the speech coding functions may be located in several alternative places, for example at the base station or in the mobile services switching center. The speech encoder and decoder are often located remote from the base station in so-called remote transcoder units. In the latter case, speech encoding parameters are sent between the base station and the transcoder unit in specific frames.
In each mobile originating or mobile terminating speech call, a transcoder is connected to the speech connection on the network side. The transcoder decodes the mobile originating speech signal (uplink direction), and encodes the mobile terminating speech signal (downlink direction). This type of an arrangement causes no-problems as long as only one of the speech participants is a mobile station and the other, for example, a subscriber of a public switched telephone network (PSTN).
If the call takes place between two mobile stations (Mobile to Mobile Call, MMC), the operation of the mobile communication network involves a transcoder on the connection between the calling mobile station and the mobile services switching center, and, correspondingly, a second transcoder between the called mobile subscriber and the (same or another) mobile services switching center. These transcoders are then inter-connected by the mobile services switching center (centers) as a result of normal call switching. In other words, for each MMC call there are two transcoder units in a series connection, and speech encoding and decoding are performed twice for the call. This is known as tandem coding. Tandem coding presents a problem in mobile communication networks as it degrades the speech quality due to the extra speech encoding and decoding. So far, tandem coding has not been a major problem because relatively few calls are MMC calls. However, as the number of mobile stations increases, the number of MMC calls will also be higher and higher.